Process for producing pulp and energy from grassland cropping systems

ABSTRACT

A novel method for producing pulp and energy from grasses is disclosed. The invention relates to producing grass plants and harvesting, handling and processing the harvested plant material.

CROSS REFERENCE

[0001] This application is related to U.S. Provisional application having serial No. 60/285,722 filed on Apr. 23, 2001 incorporated by reference herein.

FIELD OF INVENTION

[0002] This invention relates to a process for production of pulp for making paper and other fibrous products, and to bioenergy applications from grassland cropping systems. In the context of this application, a “grassland cropping system” is defined as a method of growing and producing forage grasses and other members of the grass family, such as bamboo and giant reed (Arundo donax) and grain crops, and harvesting, handling, processing, storing and transporting them to a pulp mill or energy plant. The invention further relates to a process that is based on annual and perennial grasses that are already widely grown throughout the world, primarily to produce grain and/or forage for livestock. Therefore, this invention offers the distinct advantage of diversifying use of these herbaceous crops by allowing them to be grown specifically for production of pulp and energy, in addition to their typical use for production of forage and/or grain and other purposes.

BACKGROUND OF THE INVENTION

[0003] About 92% of the world's pulp supply comes from wood (47%) and recycle paper (43%); only about 8% comes from nonwood raw material (Abramovitz and Mattoon, 1999). In the United States nonwoods account for only 0.3% of the pulp produced. Agricultural residues such as wheat straw, sugar cane bagasse and corn stalks currently make up about 75% of the world's nonwood pulp supply, while crops like kenaf and industrial hemp, that are grown specifically for pulp, make up about 9%. Bamboo makes up most of the balance. There appears to be no known case in which typical forage grasses and/or grain crops are being grown specifically to produce pulp. Some reasons for this include the following:

[0004] 1) no grassland cropping system has been developed that is economically competitive with wood-based systems for producing pulp;

[0005] 2) grasses typically have a high leaf:stem ratio, and leaves are considered to be low in fiber and unsuitable for pulp production;

[0006] 3) due to 2 above, it is necessary to remove leaves from stems if grasses are used for pulping, but this is difficult to do, adds cost to the process, and reduces usable yield per acre, thus reducing economic competitiveness;

[0007] 4) grasses typically used as forages are considered to be low in fiber and high in digestibility, and therefore unsuitable for pulp production;

[0008] 5) grasses are incompatible with wood chip feeding systems in pulp mills which process wood chips (which amount to 99.7% of the US pulping capacity);

[0009] 6) grasses have a low bulk density, and therefore are expensive to transport;

[0010] 7) grasses often do not density easily by means of pelletizing and cubing, and these densification processes are assumed to be too expensive;

[0011] 8) grasses are perceived to have higher silica contents (up to 14%) than wood, and this causes difficulty with chemical recovery in many pulping processes;

[0012] 9) expense is incurred in planting grass crops, and many growers are not willing to spend the money needed for this operation;

[0013] 10) with perennial grasses it often takes 3 years to reach full yield, and growers are often not willing to wait this long to receive a full return on their investment.

[0014] 11) grasses need to be dried after harvesting and this depends on the weather which is unsuitable in certain seasons of the year;

[0015] 12) grasses have annual growth cycles, and because of this, as well as point 11 above, they cannot be harvested continuously throughout the year, as are trees;

[0016] 13) due to points 11 and 12 above, there is a greater need to store grass material after harvesting than there is for wood;

[0017] 14) because grass material is finer and contains less lignin than wood, it does not store as well as wood, without rotting, especially in regions with high precipitation; and

[0018] 15) agricultural producers are unfamiliar with the use of grasses for production of pulp and energy, and therefore, reluctant to invest in such an enterprise.

[0019] 16) grasses are more difficult and expensive to handle, load, transport and offload than wood chips and coal are, because grasses are usually packaged in big round bales which need to be picked up, loaded and unloaded individually instead of using conveyer belt systems, as for wood chips and coal.

[0020] Most of the world's energy comes from fossil fuels: oil, coal and natural gas. It is widely recognized that biomass can be used in many processes to produce energy, including biofuels (such as ethanol and methanol), gases (such as syngas from gasification, and hydrogen), and electricity (such as co-firing biomass with coal, or running steam or gas turbines with gas generated from a biomass gasifier), but biomass is currently used in energy applications only when it is a no-cost or low-cost byproduct of another industry, and these applications have been implemented mainly with woody biomass. The pulp and paper industry is a good example: all the bark, fines and black liquor are burnt to produce energy which is used by the mill. However, there appear to be no known cases in which grassland cropping systems are being used for commercial energy production of any kind. Indeed, the reasons for this are similar to those listed above for pulping, and include the following:

[0021] 1) no grassland cropping system has been developed that can be commercially competitive with fossil fuels;

[0022] 2) grasses have a high proportion of leaves, which are considered to have a lower energy content than stems and wood;

[0023] 3) grasses are low in bulk density, and therefore expensive to transport;

[0024] 4) grasses are incompatible with coal feeding systems (they bridge and cause blockages);

[0025] 5) grasses often do not densify easily by means of pelletizing and cubing, and these densification processes are assumed to be too expensive;

[0026] 6) grasses are perceived to have higher silica contents (up to 14%) and alkali (especially potassium) contents than wood and coal, and this causes slagging and fouling of gasification and combustion systems;

[0027] 7) expense is incurred in planting grass crops, and many growers are not willing to spend the money needed for this operation; and

[0028] 8) with perennial grasses it often takes 3 years to reach full yield, and growers are often not willing to wait this long to receive a fall return on their investment.

[0029] 9) grasses need to be dried after harvesting and this depends on the weather which is unsuitable in certain seasons of the year;

[0030] 10) grasses have annual growth cycles, and because of this, as well as point 12 above, they cannot be harvested continuously throughout the year, as are trees;

[0031] 11) due to points 9 and 10 above, there is a greater need to store grass material after harvesting than there is for wood;

[0032] 12) because grass material is finer and contains less lignin than wood, it does not store as well as wood, without rotting, especially in regions with high precipitation:

[0033] 13) agricultural producers are unfamiliar with the use of grasses for production of pulp and energy, and therefore, reluctant to invest in such an enterprise; and

[0034] 14) grasses are more difficult and expensive to handle, load, transport and offload than wood chips and coal are, because grasses are usually packaged in big round bales which need to be picked up, loaded and unloaded individually instead of using conveyer belt systems, as for wood chips and coal.

SUMMARY OF THE INVENTION

[0035] The present invention comprises a method of producing a fibrous pulp comprising harvesting and drying a grass plant to produce harvested material; collecting said harvested material; size reduction of said harvested material; densification of said harvested material; and transporting said harvested material to a pulp mill to produce fibrous pulp or to an energy plant to produce energy.

[0036] Some objectives of this invention are:

[0037] 1) to provide a process for using grassland cropping systems which include annual and perennial grasses typically used for production of forage or grain, to produce pulp and energy;

[0038] 2) to provide grassland cropping systems that include perennial grasses which are already established on large areas to produce pulp and energy, thus eliminating the need and cost to plant them, and the delay in obtaining a return on these crops;

[0039] 3) to provide grassland cropping systems to produce pulp and energy in which the leaves of the grasses are not removed from the raw material, thus reducing costs;

[0040] 4) to provide grassland cropping systems that use annual and perennial grasses which are familiar to growers for producing pulp and energy, so that reluctance in investing in such enterprises is reduced;

[0041] 5) to provide grassland cropping systems that involve size reduction in the field at the site of production prior to transport of the material, to increase bulk density and reduce transport costs;

[0042] 6) to provide grassland cropping systems to produce pulp and energy that will provide a better year-round supply of raw material, thus reducing the need for storage; and

[0043] 7) to provide grassland cropping systems to produce pulp and paper that will involve densification of raw material in the field at the site of production, thus further reducing the cost of transport, increasing the compatibility of the material with coal at electricity plants and wood chips at pulp mills, and reducing losses during storage.

DETAILED DESCRIPTION OF THE INVENTION

[0044] Definitions

[0045] In the description and tables which follow, a number of terms are used. In order to provide a clear and consistent understanding of the specification and claims, including the scope to be given such terms, the following definitions are provided:

[0046] Densification is any process by which the bulk density (weight per unit volume) of a specified material is increased.

[0047] Fibrous pulp is pulp comprised of fibrous plant material. The fibers usually consist mainly of cellulose, but also include some hemicellulose, lignin and other components of plant tissue.

[0048] Size reduction refers to reduction in average particle size of particulate material as measured by the amount of material that passes through screens with different mesh sizes; e.g., half inch, quarter inch, one eighth inch, etc. For example, “long hay” (unchopped) will not pass through a screen with a mesh size of one inch, but after chopping with a forage chopper, or grinding with a tub grinder, most of the chopped material will pass through a one-inch mesh screen.

[0049] Syngas is a mixture of gases comprising primarily of carbon monoxide and hydrogen.

[0050] With respect to the above objectives and other advantages, the invention provides a process for production of pulp and energy by means of grassland cropping systems which include perennial and annual grasses that are either temperate or tropical in nature. In accordance with its more general nature, the process involves the following steps:

[0051] 1) grass species selection, including annual or perennial grasses that are either tropical or temperate in nature, to be grown alone, in rotation on the same land, or in a mixture;

[0052] 2) harvesting and drying the grass material;

[0053] 3) collecting the dried grass material;

[0054] 4) size reduction of the grass material on site in the field or at the pulp mill or energy plant, by chopping, grinding or milling;

[0055] 5) densification of the grass material on site in the field by baling, cubing or pelletizing, or at the pulp mill or energy plant by pelletizing or cubing;

[0056] 6) transporting the grass material to an energy plant or a pulp mill; and

[0057] 7) further processing of the grass material and feeding it into pulp mill or energy plant.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0058] STEP 1: Species Selection

[0059] The species used in the grassland cropping systems should preferably include one or more from the following list: Common Name Scientific Name Tropical perennials Bahiagrass Paspalum notatum Bermudagrass Cynodon dactylon Johnsongrass Sorghum halapense Panic grass Panicum maximum Switchgrass Panicum virgatum Kikuyu grass Pennisetum clandestinum Elephant grass Pennisetum purpureum Miscanthus Miscanthus gigenteus/sinensis Giant reed Arundo donax Weeping lovegrass Eragrostis curvula Tropical annuals Corn Zea mays Sorghum Sorghum vulgare Sorghum-sudangrass Sorghum almum Temperate perennials Tall fescue Festuca arundinacea Reed canary grass Phalaris arundinacea Canary grass Phalaris tuberosa Smooth brome grass Bromus inermis Perennial ryegrass Lolium perenne Temperate annuals Annual ryegrass Lolium multiflorum Wheat Triticum aestivum Rye Secale cereale Oats Avena sativa

[0060] These species can be grown alone, in rotation, and in a mixture of perennials, or by oversowing annuals into perennial sod.

[0061] STEP 2: Harvesting and Drying

[0062] Harvesting and drying can be done with regular sickle bar or disc mowers, mower conditioners, or even by hand, and harvested material can be air dried, or dried artificially.

[0063] STEP 3: Collecting Harvested Material

[0064] Harvested material can be collected with a baler or a forage chopper.

[0065] STEP 4: Size Reduction

[0066] Size reduction is achieved by chopping with a forage chopper, tub grinding, or milling/pulverizing with any of several milling/pulverizing processes.

[0067] STEP 5: Densification

[0068] Densification is achieved by baling, pelletizing, cubing, or constructing compressed grass modules, such as with a cotton module builder.

[0069] STEP 6: Transport

[0070] The material will be transported by road (truck), rail, barge or ship.

[0071] STEP 7: Further Processing and Feeding

[0072] The material may be processed further, such as co-milling grass cubes with coal, and fed into pulp mills or energy plants.

EXAMPLES

[0073] The following examples are provided to further illustrate the present invention and are not intended to limit the invention beyond the limitations set forth in the appended claims.

Example 1

[0074] Samples of grasses selected from the list above were analyzed for fiber length and diameter, heat of combustion (energy content), fusion or slagging temperature, ash content and silica content (Table 1), and some were also analyzed for cellulose, hemi-cellulose, holocellulose (cellulose+hemicellulose), lignin and usable fiber (Table 2). Clearly, fiber length and diameter of all the grasses examined are in the same general size range as those of hardwood species, indicating that all of these grasses will be suitable as hardwood substitutes for production of pulp. Unexpectedly the samples of tall fescue, bahiagrass and weeping lovegrass were comprised mainly of leaf material (over 90%), yet fiber dimensions were similar to those of hardwood fibers (Table 1).

[0075] The heat of combustion of wood and some coal is 8000-8500 BTU/lb, and again, it is surprising that values for the grasses tested also fell within this general range, even though some of the samples were comprised almost entirely of leaf material, and not stems. Furthermore, ash and silica levels are a little higher than for wood, but should be within manageable levels for pulp production, gasification, and co-firing with coal. (Table 1). TABLE 1 Preliminary test results of selected grass species for papermaking and energy compared to selected hardwoods. Fiber Fiber Heat of Fusion Length Diam. Combustion Temperature Ash Silica Species (mm) (μm) (Btu/lb) (° F.) (%) (%) Grasses Tall Fescue 0.93 11-30 8048 <1850 7.2 1.2 Bahiagrass 1.24  8-19 8168  2276 3.5 1.0 Bermudagrass 0.77 10-19 8306  2264 3.2 0.9 Weeping Lovegrass 0.96  5-10 8599 <1850 1.1 0.3 CIR Switchgrass 1.18  8-21 — — — — Alamo Switchgrass 1.37  8-18 — — — — Rye 1.08 10-23 7952 <1850 5.8 1.1 Ryegrass 1.21  8-24 8021  2048 9.8 1.0 Hardwoods Aspen* 1.04 10-27 Beech* 1.20 16-22 Oaks* 1.40 14-22 Eucalyptus** 0.80 16

[0076] Despite the grasses generally containing less cellulose than wood, they also contained less lignin, and more hemicellulose. The net result was that net usable fiber was similiar to pine (Table 2). TABLE 2 Cellulose, hemicellulose, holocellulose (cellulose + hemicellulose), lignin and usable fiber (90% of cellulose, 50% of hemicellulose and 10% of lignin which remains after the Kraft pulping process) content (%) of selected grasses and pine wood. Grass Species Cellulose Hemicellulose Holocellulose Lignin Usable fiber* Bahiagrass 35.9 33.5 69.4 8.2 49.9 Bermudagrass 34.9 36.2 71.1 9.6 50.5 Weeping lovegrass 36.7 39.9 76.6 7.3 53.7 Rye 39.2 27.6 66.8 6.0 49.7 Ryegrass 31.2 24.8 56.0 8.9 41.5 Switchgrass 36.8 33.3 70.1 7.4 50.5 Pine Southern pine 46.2 18.9 65.1 25.2 53.5

Example 2

[0077] Bulk density of chopped straw, pulverized or milled powder, cubes and pellets was measured, and the weight of these materials that could fit onto a 53×8×9 ft truck was calculated. Results are shown in Table 3. TABLE 3 Bulk density of chopped straw, pulverized powder, cubes, and pellets, and the weight of a truck load of these materials. Material Bulk density (lb/cu ft) Weight of a truck load (tons) Chopped straw  6 11.4 Milled powder 16 30.5 Cubes 25 47.7 Pellets 38 72.5

[0078] The company that makes the cubes (Warren and Baerg Manufacturing) estimates that the cost of cubing is $8.71 per ton. If it is assumed that a round trip cost from the production site to the mill is $250, then transport cost per ton for chopped straw is $250/11.4 tons $21.93, and for cubes it is $250/47.7 tons=$5.24. Therefore, the difference is $21.93−$5.24=$16.69, which more than pays for the cost of cubing. This illustrates the advantage of densification in reducing transport costs for all the methods of densification.

[0079] Although the foregoing invention has been described in some detail by way of illustration and example for purposes of clarity and understanding, it will be obvious that certain changes and modifications may be practiced within the scope of the invention, as limited only by the scope of the appended claims. 

What is claimed:
 1. A method of producing a fibrous pulp comprising: 1) harvesting and drying a grass plant to produce harvested material; 2) collecting said harvested material; 3) size reduction of said harvested material; 4) densification of said harvested material; and 5) transporting said harvested material to a pulp mill to produce fibrous pulp, or to an energy plant to produce energy.
 2. The method of claim 1, wherein said grass plant is selected from the group consisting of bahiagrass, bermudagrass, johnsongrass, panic grass, switchgrass, kikuyu grass, elephant grass, Miscanthus, giant reed, weeping lovegrass, corn, sorghum, sorghum-sudangrass, tall fescue, reed canary grass, canary grass, smooth brome grass, perennial ryegrass, bamboo, Arundo, annual ryegrass, wheat, rye, or oats.
 3. The method of claim 1, wherein said densification is done by baling the harvested material into round or square bales.
 4. The method of claim 3, wherein said bales are ground in a tub grinder to produce pieces of grass.
 5. The method of claim 4, wherein said pieces are densified by building a module with a cotton module builder.
 6. The method of claim 4, wherein said densification is carried out by compressing the piecess or powder into pellets or cubes having a density range of about 10 lbs/ft³ to about 90 lbs/ft³
 7. The method of claim 1, wherein said size reduction is done by chopping said plant into pieces.
 8. The method of claim 7, wherein said pieces are densified by building a module with a cotton module builder.
 9. The method of claim 7, wherein said densification is carried out by compressing the pieces or powder into pellets or cubes having a density range of about 10 lbs/ft³ to about 90 lbs/ft³.
 10. The method of claim 1 wherein said size reduction is done by milling or pulverizing said plant into a powder with a bulk density between about 10 lbs/ft³ and about 60 lb/ft³.
 11. The method of claim 10, wherein said pieces are densified by building a module with a cotton module builder.
 12. The method of claim 10, wherein said densification is carried out by compressing the pieces or powder into pellets or cubes having a density range of about 10 lbs/ft³ to about 90 lbs/ft³.
 13. The method of claim 6, wherein the pellets or cubes are from {fraction (1/2)} inch in length to about 10 inches in length.
 14. The method of claim 13, wherein the pellets or cubes are fed into a energy plant to produce energy.
 15. The method of claim 14, wherein the said cubes or pellets are co-milled with coal prior to co-firing the harvested material with coal to produce electricity.
 16. The method of claim 14, wherein the said pellets or cubes are fed into a pulp mill to produce a fibrous pulp.
 17. The method of claim 14, wherein said pellets or cubes are fed into a pulp mill in a mixture with wood chips to produce a fibrous pulp.
 18. The method of claim 10, wherein said powder is fed into a gasifier to produce syngas. 